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band_pass_hovmueller

Create a band-pass filtered time series suitable for a time vs. longitude (Hovmueller) plot.

Available in version 5.1.0 and later.

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"        ; These three libraries are automatically
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"         ; loaded from NCL V6.2.0 onward.
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"     ; No need for user to explicitly load.

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/diagnostics_cam.ncl" ; Still need to load this library.

	function band_pass_hovmueller (
		x [*][*][*] : numeric,  
		srate   [1] : numeric,  
		bpf     [3] : integer,  
		wgty    [*] : numeric,  
		opt     [1] : logical   
	)

	return_val  :  array

Arguments

A three-dimensional variable array. Nominally: (time,lat,lon).

srate

Sampling rate. For example: if srate refers to "samples per day", then, srate is one of the following [1,2,4,8,12,24]. These correspond to [24,12,6,3,2,1] hour sampling rates. Another common sampling unit is "samples per year".

bpf

An integer array of length 3. The first two elements contain the band pass start and end times. The last element is the number of weights to be used.

wgty

A one-dimensional array containing the latitudinal weights.

opt

If opt=False, the function will operate under default mode regardless of any attributes associated with the variable.

If opt=True, then attributes associated with opt may be used to alter the default behavior.

opt@detrend=True: Series will be detrended. Default: opt@detrend=False.

Return value

A two-dimensional array [nominally: (time,lon) ] containing areal weighted and band-passed filtered values.

The following attributes will be associated with the returned variable.

band_pass_start=bpf(0)

band_pass_last=bpf(1)

band_pass_Nwgts=bpf(2)

var_0, var_1, var_2= string identifiers

Description

At each time step, weighted latitudinal averages are computed using dim_avg_wgt. The resulting array is band-pass filtered in time using Lanczos weights (filwgts_lanczos). The user must be aware that the appropriate number of filter weights is a direct function of the band pass period. The shorter the period, the more weights that will be required to sufficiently resolve the desired period.

The input array, x, should have sufficient 'temporal padding' so that the filter will have its desired result over the period of interest. For example, if daily mean data, srate=1 (samples per day), are being analyzed and bpf(2)=201, then the input array should have 100 (=bpf(2)/2) days padding before and after the time period of focus.

FYI: The US-CLIVAR MJO working group:

           http://www.usclivar.org/mjo.php

has suggested a number of diagnostics for evaluating the MJO ( Madden-Julian Oscillation). See their diagnostics website:

     http://climate.snu.ac.kr/mjo_diagnostics/index.htm

This inclues the statement: "Intraseasonal (20-100 day) bandpass filtered anomalies are constructed using a 201-point Lanczos filter, which has half power points at 20 day and 100 day periods."

Also,

"No windowing/tapering or de-trending was applied in the calculation of these spectra, since sensitivity tests indicated their application had a negligible impact on the results"

Examples

The following example focuses on the Madden-Julian Oscillation [MJO]. However, this function is general purpose and can be applied to any spatial time series. It can readily be used to focus on other periods of interest.

Example 1

Consider daily "olr". Extract MJO (Madden-Julian Oscillation) information using the 20-to-100 day band pass period and 201 weights as suggested by the WMO CLIVAR Working Group.

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/diagnostics_cam.ncl"


  f     = addfile("...", "r") 
  x     = f->OLR                 ; (time,lat,lon) , ntim=1461

  latS  =  -5.              ; select region
  latN  =   5.
  lonL  =  60.              ; Indian Ocean
  lonR  = 280.              ; East Pacific

  srate = 1                 ; daily means
  bpf   = (/20,100,201/)    ; MJO ... suggested by MJO WG [WMO] 
  wgty  = latRegWgt(x&lat, "double", 0) 
  opt      = False
                            ; pass in area of interest
  mjo_hov = band_pass_hovmueller (x(:,{latS:latN},{lonL:lonR} \
                               ,srate, bpf, wgty({latS:latN}), opt)

  printVarSummary( mjo_hov )
  printMinMax( mjo_hov, True)

The edited output (currently) looks like: